<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        * {
            padding: 0;
            margin: 0;
        }
        
        table {
            border-collapse: collapse;
            border-spacing: 0;
            border: 1px solid #c0c0c0;
            width: 300px;
            margin: 100px auto;
        }
        
        th,
        td {
            border: 1px solid #d0d0d0;
            color: #404060;
            padding: 10px;
        }
        
        th {
            background-color: #09c;
            font: bold 16px "微软雅黑";
            color: #fff;
        }
        
        td {
            font: 14px "微软雅黑";
        }
        
        tbody tr {
            background-color: #f0f0f0;
        }
        
        tbody tr:hover {
            cursor: pointer;
            background-color: #fafafa;
        }
    </style>
</head>

<body>
    <table>
        <thead>
            <tr>
                <th><input type="checkbox" id="checkAll"></th>
                <th>商品</th>
                <th>价格</th>
            </tr>
        </thead>
        <tbody id="j_tb">
            <tr>
                <td>
                    <input type="checkbox" class="follow" />
                </td>
                <td>iPhone8</td>
                <td>8000</td>
            </tr>
            <tr>
                <td>
                    <input type="checkbox" class="follow" />
                </td>
                <td>iPad Pro</td>
                <td>5000</td>
            </tr>
            <tr>
                <td>
                    <input type="checkbox" class="follow" />
                </td>
                <td>iPad Air</td>
                <td>2000</td>
            </tr>
            <tr>
                <td>
                    <input type="checkbox" class="follow" />
                </td>
                <td>Apple Watch</td>
                <td>2000</td>
            </tr>

        </tbody>
    </table>

    <script>
        var All = document.querySelector('#checkAll');
        var Fol = document.querySelectorAll('.follow');
        // 1.fol跟随all状态
        All.onclick = function() {
                console.log(this.checked);
                for (i = 0; i < Fol.length; i++) {
                    Fol[i].checked = this.checked;
                }
            }
            // 2.当所有fol=1,all=1,当有一个fol=0,all=0
            // 思路,遍历所有fol
        for (i = 0; i < Fol.length; i++) {
            Fol[i].onclick = function() {
                for (j = 0; j < Fol.length; j++) {
                    if (Fol[j].checked == 1) {
                        All.checked = 1
                    } else if (Fol[j].checked == 0) {
                        All.checked = 0;
                        break;
                    }
                }

            }

        }
    </script>
</body>

</html>